The blowing of foil utilizing a vertical blowing head generally requires the extrusion of a thin layer of thermoplastic melt between a core and a surrounding portion of an extrusion die so that a tubular plastic structure emerges from the die and can be expanded at the mouth of the die by a cooling fluid which is introduced into the tubular structure through the die and can be conducted via passages in the die from the inflated interior of the tube. The end of the tube is closed at a location spaced from the die so that the degree of expansion is a function of the pressure of the fluid and its flow rate.
During expansion the relatively thick walls of the tubular structure are stretched and thereby reduced in thickness to produce comparatively thin foils. The tubular foil web which is thereby produced can be flattened, slit, if desired, and/or fabricated into bags or other products.
A vertically oriented foil-blowing head of this type, i.e. for the production of thin-walled tubular foils of thermoplastic material, can comprise a stationary lower part provided with a connection to an extrusion worm or plasticizing press in which liquefaction of the thermoplastic material occurs and from which the thermoplastic is fed through an annular gap of the die to produce the tubular structure.
The die itself generally comprises a pair of members, namely, the core and the outer part, i.e. the ring, which define the annular gap and which are rotated.
A rotating feeder is provided for introducing the cooling and blowing air and discharges the cooling and blowing air, this device including passages extending through the core. A distributor is generally also used to free the thermoplastic material from the stationary inlet at which the head is connected to the extrusion press, at the annular passages from which the tubular structure emerges for blowing downstream of the movement of the die.
In earlier constructions of this type, the distributor had a central melt inlet passage and a manifold system communicating with the annular passage while the air feeder included an annular passage encircling the head.
This arrangement allowed the die and the air supply and discharge device to rotate about the axis of the head and hence distribute any irregularities in the thickness substantially uniformly around this axis. In practice, the distributor passage system either comprised a distributor cone or a number of distributor passage sections which were spaced apart around the perforation of the melt flow passage.
A major problem has been discovered with such systems, namely, the excessive and nonuniform cooling of the melt before it enters the annular passage in the die and certainly before it emerges therefrom, when, as in earlier systems, the cooling air passage has traversed the distributor for the melt.
This undesired cooling results in a texturing of the product and the formation of lines or rays in the blown foil which can be attributed to localized nonuniform cooling and which are detrimental to the appearance and mechanical strength of the product.